What is chlorination in Sewage Treatment Plant?

The most common and successful disinfection process in Sewage Treatment Plant is chlorination. Chlorination is performed through the addition of chlorine or chlorinated substances to treated water with the aim of killing pathogenic microbes prior to disposal into the environment or for recycling. Chlorination is an important process that renders the effluent of an STP harmless and free from causing any damage to human life or aquatic life.

We will be explaining in detail the significance of chlorination in STPs, how it is being performed, what kind of chlorine is utilized, advantages and disadvantages of the same.

Why Chlorination is Needed in STPs?

Treated sewage wastewater from sewage treatment plants discharged is usually contaminated with bacteria, viruses, and other waterborne disease-causing microorganisms. Water, if discharged without undergoing the disinfection process, becomes a health hazard to man and causes environmental pollution. Chlorination is necessary to:

· Killed disease-causing microorganisms such as bacteria.

· Disease prevention such as cholera, typhoid, and dysentery.

· Odor control through decomposition of organic matter.

· Reduction of the risk of natural water bodies becoming contaminated when treated water is discharged into them.

Without chlorination, untreated or even partially treated sewage can cause serious public health issues and environmental degradation.

How Does Chlorination Work in an STP?

Chlorination in an STP would be secondary or tertiary treatment where the majority of organic matter and suspended solids would already have been removed. Chlorine is used on pre-treated water and microorganisms respond by destroying them by destroying their cell structure so that they are not able to replicate or infect.

Chlorination efficiency is susceptible to a vast range of conditions including:

· Chlorine dose: Quantity of chlorine added to water.

· Contact time: Duration for chlorine to remain in water so that it can sanitize.

· Water quality: Organic load, pH, and temperature affect the effectiveness of chlorine.

Once chlorination is complete, a process called dechlorination may be required to remove excess chlorine before the water is released into the environment. This prevents harmful effects on aquatic ecosystems.

Types of Chlorination in STPs

There are various chlorination methods employed in STPs depending on the demand and safety factor.

Liquid Chlorination (Sodium Hypochlorite)

Sodium hypochlorite (NaOCl), or liquid bleach, is one of the most commonly used disinfectants applied in STPs. It is easy to store and handle but has a short shelf life. It is generally dosed into water by means of dosing pumps so that it is evenly distributed.

Gas Chlorination (Chlorine Gas)

Chlorine gas (Cl?) is a potent disinfectant and is used in most of the large sewage works. It is kept under pressure as liquid in cylinders and released into water in a regulated system of dosing. Gas chlorination is cost-effective but requires strict safety precautions as it is poisonous.

Solid Chlorination (Calcium Hypochlorite)

Calcium hypochlorite (Ca(ClO)?) is also available in tablet or granular form. It can be used in small sewage treatment plants (STPs) or where liquid and gaseous chlorination is not convenient. It is soluble in water and releases chlorine for effective disinfection.

Advantages of STP Chlorination

Chlorination is implemented on a large scale in the sewage treatment plant because it has many benefits:

· Effective Disinfection: Chlorine is capable of effectively disinfecting various pathogens, removing wastewater's potential to be treated.

· Low Cost: Chlorination is cheaper than most other forms of disinfection.

· Simple Installation: The process is easy to install and does not need very highly sophisticated machinery.

· Residual Action: Chlorine remains in water for a certain time to continue acting as a disinfectant before it gets flushed out.

Disadvantages of Chlorination in STPs

There are, nonetheless, limitations to the chlorination that have to be handled with too much care:

· Toxic Byproduct Formation: While reacting with organic compounds, chlorine can form toxic byproducts like trihalomethanes (THMs) that are bound to be toxic.

· Environmental Issues: Too much chlorine not divided from the rest, if emitted, is able to destroy aquatic life.

· Storage and Handling Risks: Chlorine gas is a harmful chemical that must have appropriate safety precautions in storage and handling to prevent leakages and exposure.

· Dependence on the Quality of Water: Efficiency can be reduced by excessive organic matter or specific levels of pH during chlorination.

To address these challenges, many STPs use dechlorination methods such as adding sodium bisulfite to neutralize excess chlorine before releasing the treated water into the environment.

Conclusion

Chlorination is an essential operation in STPs, in the way that the treated sewage water is made microbially sterile prior to its discharge into natural water bodies or reuse. It is economical, efficient, and standard practice of disinfection by STPs, which avoids water-borne diseases and saves public health.

However, proper handling and monitoring of chlorine use are necessary to prevent the formation of harmful byproducts and minimize environmental impact. With the right practices, chlorination remains one of the most reliable ways to ensure the safe and sustainable treatment of wastewater.

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